1. Field of Invention
This invention relates to an electronic linear scale. More particularly, this invention is directed to electronic linear scales using a reduced offset high accuracy induced current position transducer.
2. Description of Related Art
U.S. patent application Ser. No. 08/645,490, filed May 13, 1996, and incorporated herein in its entirety, discloses an electronic linear scale using an inductive position transducer. The '490 application discloses signal processing techniques for induced current position transducers.
The operation of the electronic linear scale using the inductive position transducer described in the '490 application is generally shown in FIGS. 1, 2, and 3. As shown in FIG. 1, an electronic linear scale 100 includes an elongated beam 102 and a slider assembly 120. The beam 102 is a rigid or semi-rigid bar having a generally rectangular cross section.
During operation, the slider assembly 120 is installed in close proximity to scale 104 without contacting any components on the beam 102 itself. Both the beam 102 and the slider assembly 120 may be affixed to other objects in a variety of configurations. In this manner, the linear scale 100 measures the position of the slider assembly 120 relative to the beam 102, thereby also measuring the relative position between any mechanical fixtures to which these two components are attached.
Ultimately, the scale 104 may be fabricated of a flexible or rigid dimensionally stable material. The scale 104 can be applied directly to a mechanical fixture with clamps or adhesive, thus eliminating the beam 102. The scale 104 and the beam 102 are preferably, but not necessarily, nonconductive. The slider assembly 120 inductively monitors its own position relative to the scale 104.
The scale 104 is an elongated printed circuit board 106. As shown in FIG. 1, a set of magnetic flux modulators 108 are spaced apart along the printed circuit board 106 in a periodic pattern. A display cable 110 is also shown in FIG. 1.
As shown in FIG. 2, the measurement generated by the linear scale is displayed on a conventional digital display 112 mounted on the digital display unit 114. A pair of pushbutton switches 116 and 118 are also mounted on the digital display unit 114. The switch 116 turns on and off the signal processing electronics 122 and the digital display unit 114. The switch 118 resets the origin of the slider assembly 120 to its current position relative to the scale 104. Alternately, the position information from the slider assembly 120 may be routed to other types of electronic control systems or displays. The slider assembly 120 includes a slider housing 124, which holds the read head 130 in close proximity to the flux modulators 108 on the beam 102 without making contact.
The slider assembly 120 also includes a substrate 126, such as a conventional printed circuit board. The signal processing electronics 122 are preferably mounted on an upper surface of the substrate 126, although a portion of the signal processing electronics may be placed on the interior side of the read head 130, as an alternative (not shown). The contents of the slider housing 124 are protected by a cover 128. A ribbon-like read head connector 132 electrically connects the read head 130 to the signal processing electronics 122. The display cable 110 is attached to the signal processing electronics 122 inside the housing 124 by conventional means.
As shown in FIG. 3, the plurality of magnetic flux modulators 108 are distributed along a measuring axis 134 of the elongated beam 102 at a pitch equal to a wavelength .lambda., which is described in more detail below. The flux modulators 108 have a nominal width along the measuring axis 134 of .lambda./2. The flux modulators 108 have a width d in a direction perpendicular to the measuring axis 134.
The read head 130 includes a generally rectangular transmitter winding 136 that is connected to a drive signal generator 138. The drive signal generator 138 provides a time-varying drive signal to the transmitter winding 136. The time-varying drive signal is preferably a high frequency sinusoidal signal, a pulse signal or an exponentially decaying sinusoidal signal. When the time-varying drive signal is applied to the transmitter winding 136, the time-varying current flowing in the transmitter winding 136 generates a corresponding time-varying, or changing, magnetic field. Because the transmitter winding 136 is generally rectangularly shaped, the generated magnetic field is substantially constant within a flux region in the central portion of the transmitter winding 136.
The read head 130 further includes a first receiver winding 140 and a second receiver winding 142 positioned on the read head 130 within the flux region inside the transmitter winding 136. Each of the first receiver winding 140 and the second receiver winding 142 is formed by a plurality of first loop segments 144 and second loop segments 146. The first loop segments 144 are formed on a first surface of a layer of the printed circuit board 130. The second loop segments 146 are formed on another surface of the layer of the printed circuit board 130. The layer of the printed circuit board 130 acts as an electrical insulation layer between the first loop segments 144 and the second loop segments 146. Each end of the first loop segments 144 is connected to one end of one of the second loop segments 146 through feed-throughs 148 formed in the layer of the printed circuit board 130.
The first and second loop segments 144 and 146 are preferably sinusoidally shaped. Accordingly, as shown in FIG. 3 the first and second loop segments 144 and 146 forming each of the receiver windings 140 and 142 form a sinusoidally shaped periodic pattern having a wavelength B. Each of the receiver windings 140 and 142 are thus formed having a plurality of loops 150 and 152.
The loops 150 and 152 in each of the first and second receiver windings 140 and 142 have a width along the measuring axis 134 equal to .lambda./2. Thus, each pair of adjacent loops 150 and 152 has a width equal to .lambda.. Furthermore, the first and second loop segments 144 and 146 go through a full sinusoidal cycle in each pair of adjacent loops 150 and 152. Thus, .lambda. corresponds to the sinusoidal wavelength of the first and second receiver windings 140 and 142. Furthermore, the second receiver winding 142 is offset by .lambda./4 from the first receiver winding 140 along the measuring axis 134. That is, the first and second receiver windings 140 and 142 are in quadrature.
The changing drive signal from the drive signal generator 138 is applied to the transmitter winding 136 such that current flows in a transmitter winding 136 from a first terminal 136a, through the transmitter winding 136 and out through a second terminal 136b. Thus, the magnetic field generated by the transmitter winding 136 descends into the plane of FIG. 3 within the transmitter winding 136 and rises up out of the plane of FIG. 3 outside the transmitter winding 136. Accordingly, the changing magnetic field within the transmitter winding 1 generates an induced electromagnetic force (EMF) in each of the loops 150 and 152 formed in the receiver windings 140 and 142.
The loops 150 and 152 have opposite winding directions. Thus, the EMF induced in the loops 150 has a polarity that is opposite to the polarity of the EMF induced in the loops 152. The loops 150 and 152 enclose the same size areas and thus nominally the same amount of magnetic flux. Therefore, the absolute magnitude of the EMF generated in each of the loops 150 and 152 is nominally the same.
There are preferably equal numbers of loops 150 and 152 in each of the first and second receiver windings 140 and 142. Ideally, the positive polarity EMF induced in the loops 150 is exactly offset by the negative polarity EMF induced in the loops 152. Accordingly, the net nominal EMF on each of the first and second receiver windings 140 and 142 is zero. Thus no signal should be output from the first and second receiver windings 140 and 142 as a result solely of the direct coupling from the transmitter winding 136 to the receiver windings 140 and 142.
When the read head 130 is placed in proximity to the scale 104, the changing magnetic flux generated by the transmitter winding 136 also passes through the flux modulators 108. The flux modulators 108 modulate the changing magnetic flux and can be either flux enhancers or flux disrupters.
When the flux modulators 108 are provided as flux disrupters, the flux modulators 108 are formed as conductive plates or thin conductive films on the scale 104. As the changing magnetic flux passes through the conductive plates or thin films, eddy currents are generated in the conductive plates or thin films. These eddy currents in turn generate magnetic fields having a field direction that is opposite to that of the magnetic field generated by the transmitter winding 136. Thus, in areas proximate to each of the flux disrupter-type flux modulators 108, the net magnetic flux is less than the net magnetic flux in areas distant from the flux disrupter type flux modulators 108.
When the scale 104 is positioned relative to the read head 130 such that the flux disrupters 108 are aligned with the positive polarity loops 150 of the receiver winding 140, the net EMF generated in the positive polarity loops 150 is reduced compared to the net EMF generated in the negative polarity loops 152. Thus, the receiver winding 140 becomes unbalanced and has a net negative signal across its output terminals 140a and 140b.
Similarly, when the flux disrupters 108 are aligned with the negative polarity loops 152, the net magnetic flux through the negative polarity loops 152 is disrupted or reduced. Thus, the net EMF generated in the negative polarity loops 152 is reduced relative to the net EMF generated in the positive polarity loops 150. Thus, the first receiver winding 140 has a net positive signal across its output terminals 140a and 140b.
When the flux modulators 108 are provided as flux enhancers, this result is exactly reversed. The flux enhancer type flux modulators 108 are formed by portions of high magnetic permeability material provided in or on the scale 104. The magnetic flux generated by the transmitter winding 136 preferentially passes through the high magnetic permeability flux enhancer type flux modulators 108. That is, the density of the magnetic flux within the flux enhancers 108 is enhanced, while the flux density in areas outside the flux enhancers 108 is reduced.
Thus, when the flux enhancers 108 are aligned with the positive polarity loops 150 of the second receiver winding 142, the flux density through the positive polarity loops 150 is greater than a flux density passing through the negative polarity loops 152. Thus, the net EMF generated in the positive polarity 150 increases, while the net EMF induced in the negative polarity loops 152 decreases. This appears as a positive signal across the terminals 142a and 142b of the second receiver winding 142.
When the flux enhancers 108 are aligned with the negative polarity loops 152 of the second receiver winding 142, the negative polarity loops 152 generate an enhanced EMF relative to the EMF induced in the positive polarity loops 150. Thus, a negative signal appears across the terminals 142a and 142b of the second receiver winding 142. It should also be appreciated that, as outlined in the incorporated reference, both the flux enhancing and flux disrupting effects can be combined in a single scale, where the flux enhancers and flux disrupters are interleaved along the length of the scale 104. This would act to enhance the modulation of the induced EMF, because the effects of both types of flux modulators additively combine.
As indicated above, the width and height of the flux modulators 108 are nominally .lambda./2 and d, respectively, while the pitch of the flux modulators 108 is nominally .lambda.. Similarly, the wavelength of the periodic pattern formed in the first and second receiver windings 140 and 142 is nominally .lambda. and the height of the loops 150 and 152 is nominally d. Furthermore, each of the loops 150 and 152 enclose nominally the same area.
FIG. 4A shows the position-dependent output from the positive polarity loops 150 as the flux modulators 108 move relative to the positive polarity loops 150. Assuming the flux modulators 108 are flux disrupters, the minimum signal amplitude corresponds to those positions where the flux disrupters 108 exactly align with the positive polarity loops 150, while the maximum amplitude positions correspond to the flux disrupters 108 being aligned with the negative polarity loops 152.
FIG. 4B shows the signal output from each of the negative polarity loops 152. As with the signal shown in FIG. 4A, the minimum signal amplitude corresponds to those positions where the flux disrupters 108 exactly align with the positive polarity loops 150, while the maximum signal output corresponds to those positions where the flux disrupters exactly align with the negative polarity loops 152. It should be appreciated that if flux enhancers were used in place of flux disrupters, the minimum signal amplitudes in FIGS. 4A and 4B would correspond to the flux enhancers 108 aligning with the negative polarity loops 152, while the maximum signal amplitude would correspond to the flux enhancers 108 aligning with the positive polarity loops 150.
FIG. 4C shows the net signal output from either of the first and second receiver windings 140 and 142. This net signal is equal to the sum of the signals output from the positive and negative polarity loops 150 and 152, i.e., the sum of the signal shown in FIGS. 4A and 4B. The net signal shown in FIG. 4C should ideally be symmetrical around zero, that is, the positive and negative polarity loops 132 and 134 should be exactly balanced to produce a symmetrical output with zero offset.
However, a "DC" (position independent) component often appears in the net signal in a real device. This DC component is the offset signal V.sub.o. This offset V.sub.o is an extraneous signal component that complicates signal processing and leads to undesirable position measurement errors. This offset has two major sources.
First, the full amplitude of the transmitter field passes through the first and second receiver windings 140 and 142. As outlined above, this induces a voltage in each loop 150 and 152. The induced voltage nominally cancels because the loops 150 and 152 have opposite winding directions. However, to perfectly cancel the induced voltage in the receiver windings requires the positive and negative loops 150 and 152 to be perfectly positioned and shaped, for a perfectly balanced result. The tolerances on the balance are critical because the voltages induced directly into the receiver winding loops 140 and 142 by the transmitter winding 136 are much stronger than the modulation in the induced voltage caused by the flux modulators 108. In practice, fabrication tolerances always prevent perfect balance.
Second, the spatially modulated field created by the flux modulators 108 also exhibits an average position-independent offset component. That is, the flux modulators 108 within the magnetic field generated by the transmitter winding 136 all create the same polarity spatial modulation in the magnetic field. For example, when flux disrupters are used, the induced eddy current field from the flux modulators has an offset because the flux disrupters within the transmitter field all create a same polarity secondary magnetic field. At the same time, the space between the flux disrupters does not create a secondary magnetic field.
Thus, each positive polarity loop 150 and each negative polarity loop 152 of the receiver windings 140 and 142 sees a net magnetic field that varies between a minimum value and a maximum value having the same polarity. The mean value of this function is not balanced around zero, i.e., it has a large nominal offset. Similarly, when flux enhancers are used, the field modulation due to the flux enhancers has an offset because the enhancers within the transmitter winding 136 all create the same field modulation, while the space between the modulators provides no modulation. Each positive and negative polarity loop 150 and 152 of each receiver winding 140 or 142 therefore sees a spatially modulated field that varies between a minimum value and a maximums. value having the same polarity. The mean value of this function also has a large nominal offset.
A receiver winding having an equal number of similar positive and negative polarity loops 150 and 152 helps eliminate the offset components. However, any imperfection in the balance between the positive and negative polarity loops 150 and 152 allows residual offsets according to the previous description.
Both these offset components are expected to be canceled solely by the symmetry between the positive and negative polarity loops 150 and 152 in the first and second windings 140 and 142. This puts very stringent requirements on the manufacturing precision of the receiver windings 140 and 142. Experience in manufacturing a transducer indicates it is practically impossible to eliminate this source of error from the induced current position transducer of a conventional linear scale.
Furthermore, any deviations in the width or pitch of the flux modulators 108 will unbalance the receiver windings 140 or 142 in a way that is independent of the relative position between the scale 104 and the slider assembly 120.
Any signal component which is independent of the transducer position, such as the aforementioned offset components, is regarded as an extraneous signal to the operation of the transducer. Such extraneous signals complicate the required signal processing circuitry and otherwise lead to errors which compromise the accuracy of the transducer.
One proposed solution attempts to reduce the extraneous coupling between the transmitter and receiver windings simply by placing the receiver winding distant from the field produced by the transmitter winding. However, the effectiveness of this technique alone depends on the degree of separation between the transmitter and receiver windings. Hence, this technique contradicts the need for high accuracy linear scale of compact size. Alternatively, the transmitter field can be confined with magnetically permeable materials so that the effectiveness of a given degree of separation is increased. However, this technique leads to additional complexity, cost, and sensitivity to external fields, in a practical device.
Furthermore, the simple winding configurations disclosed in association with these techniques include no means for creating a device with a measuring range significantly exceeding the span of the transmitter and receiver windings. In addition, the simple winding configurations provide no means for significantly enhancing the degree of output signal change per unit of displacement for a given measuring range. Thus, the practical measuring resolution of these devices is limited for a given measuring range.
The need for a high accuracy inductive linear scale which rejects both extraneous signal components and external fields, is compact, of simple construction, and capable of high resolution measurement over an extended measuring range without requiring increased fabrication and circuit accuracies, has therefore not been met previously.